In electronic equipment, there is a need for electrical connectors providing connections in signal paths, and often the signal paths are so closely spaced that difficulties arise from interference between signals being transmitted along adjacent paths.
In order to minimize such difficulties it is known to provide grounding connections in such connectors, such connections serving in effect to filter out undesired interference between signal paths.
However, mere grounding is not always sufficient, and this is particularly so in connectors in which contacts constituting the signal paths through the connector extend through sharp angles, because interference between adjacent signal paths is a particularly large problem in such connectors.
In many situations where electrical signals are being carried among separate subassemblies of complex electrical and electronic devices, reduced size contributes greatly to the usefulness or convenience of the devices or of certain portions of them. To that end, cables including extremely small conductors are now available, and it is practical to manufacture very closely spaced terminal pads accurately located on circuit boards or the like. It is therefore desirable to have a connector of reduced size, to interconnect such cables and circuit boards repeatedly, easily, and reliably, and with a minimum adverse effect on electrical signal transmission in a circuit including such a connector.
In high speed backplane applications, low crosstalk between signal currents passing through the connector is desirable. Additionally, maximizing signal density is also desirable. Low crosstalk insures higher signal integrity. High density increases the number of circuits that can be routed through the connector.
Pin and socket type connectors are typically used to achieve a disconnectable, electrically reliable interface. Moreover, reliability is further increased by providing two redundant, cantilever-type points of contact. Conventional approaches typically locate two receptacle cantilever beams on opposing sides of a projecting pin or blade. This 180.degree. "opposing-beam" method requires a significant amount of engagement clearance in the plane that is defined by the flexing movement of the cantilever beams during engagement. Additionally, due to manufacturing tolerances, end portions of the beams are angled outward from the center lengthwise axis of a mating pin or blade in order to prevent stubbing during initial engagement. This clearance for spring beam flexure and capture projections creates a requirement for contact clearance in the "flexing plane". This clearance must be accommodated in the connector receptacle housing, thereby becoming a significant limiting factor in improving connector density.
To achieve minimum crosstalk through a coaxial-like isolation of the signal current passing within the connector, isolation in both vertical and horizontal planes alongside the entire connector signal path (including the engagement area) is desired. Clearance requirements in the opposing cantilever beam flexing plane conflicts with requirements for vertical and horizontal electrical isolation while simultaneously maintaining or increasing connector density.
A method for achieving electrical isolation with use of an "L-shaped" ground contact structure is described in a U.S. patent issued to Sakurai (U.S. Pat. No. 5,660,551). Along the length of the receptacle connector, Sakurai creates an L-shape within the cross-section of the ground contact body. In the contact engagement means area, Sakurai transitions to a flat, conventional dual cantilever beam receptacle ground contact and relies on a 90.degree. rotated flat projecting blade, thereby producing an L-shape cross-section when the blade and the receptacle are engaged. This transition of the L-shaped structure in the contact engagement section limits density due to the above described flexing-plane clearance concerns with both the signal and ground dual-beam contacts and also creates an opportunity for producing gap sections where full coaxial-like isolation cannot be maintained. Moreover, in Sakurai, all four cantilever beams flexing planes are oriented in parallel fashion, thereby limiting density.
One conventional method of transmitting data along a transmission line is the common mode method, which is also referred to as single ended. Common mode refers to a transmission mode which transmits a signal level referenced to a voltage level, preferably ground, that is common to other signals in the connector or transmission line. A limitation of common mode signaling is that any noise on the line will be transmitted along with the signal. This common mode noise most often results from instability in the voltage levels of the common reference plane, a phenomenon called ground bounce.
Another conventional method of transmitting data along a transmission line is the differential mode method. Differential mode refers to a method where a signal on one line of voltage V is referenced to a line carrying a complement voltage of -V. Appropriate circuitry subtracts the lines, resulting in an output of V-(-V) or 2V. Any common mode noise is canceled at the differential receiver by the subtraction of the signals.
Implementation of differential pairing in a high speed right angle backplane connectors is typically column-based because shields at ground potential are inserted between the columns of contacts within the connector. In other words, in order to improve signal integrity, conventional products typically use a column-based pair design, such as that found in the VHDM products manufactured by Teradyne, Inc. of Boston, Mass. In column-based pairing, skew is introduced between the true and complement voltages of the differential pair. One of the pair of signals will arrive sooner than the other signal. This difference in arrival time degrades the efficiency of common mode noise rejection in the differential mode and slows the output risetime of the differential signal. Thus, because bandwidth, which is a measure of how much data can be transmitted through a transmission line structure, is inversely related to the length of the risetime by Bandwidth=0.35/Risetime, the amount of the data throughput is degraded by column-based pairing.
Although the art of electrical connectors is well developed, there remain some problems inherent in this technology, particularly densely packing contact members while preventing crosstalk between adjacent contact members. Therefore, a need exists for electrical connectors that have small footprints while maintaining signal integrity.